Various forms of multi-layer plastic films are well-known in the art. For example, Schirmer U.S. Pat. No. 3,832,270, entitled "Heat Shrinkable, Oriented Laminated Plastic Film", discloses a multi-layer package wrapping film. The film of the Schirmer patent is a multi-layer laminate structure with a layer of ethylene-vinyl acetate bonded to a layer of polyethylene. Prior to the lamination step, the layer of ethylene-vinyl acetate is irradiated to effect cross-linking.
Further, Nakamura et al U.S. Pat. No. 4,465,487, entitled "Container for Medical Use", discloses a plastic container formed of ethylene-vinyl acetate. The collapsible container is formed of a single layer sheet material. This material has a preferred thickness in a range of 0.010-0.015 inch. The container is first heat sealed at its edges to provide fluid resistant seals. Then, the container is irradiated to provide desired cross-linking in the material to provide for heat resistance during autoclaving. Also in the Nakamura et al patent, a port is affixed to the body of the container after cross-linking has taken place, using non-cross-linked meltable plastic as an adhesive.
Poly(ethylene-vinyl acetate) materials, often referred to as EVA materials, are sufficiently flexible to form containers usable in the medical field. One advantage of EVA materials is that they do not require plasticizers, as do polyvinyl chloride materials. As a result, the quantity of leachable materials or extractable components can be essentially eliminated from the plastic film. Further, prior to cross-linking, the EVA material is easily heat-sealable employing, for example, dielectric heating methods using radio frequency voltages between metal heat sealing discs.
However, known conventional containers made from EVA materials have certain distinct disadvantages. For example, when subjected to pasteurizing or autoclaving temperatures on the order of 110.degree. C. or more, the EVA materials tend to soften, distort or melt. The softening effect can be inhibited by high energy irradiation of the EVA material to provide the necessary and desirable cross-linking, thus improving the heat-resistant characteristics of the EVA material. However, all heat-sealing of the material to provide fluid resistant seals must be performed prior to the irradiation step.
In addition, known conventional EVA containers tend to have wall thicknesses on the order of 0.015 inch. This wall thickness has been conventionally utilized in connection with radio frequency, dielectric heat sealing, because it results in acceptable heat sealed joints with minimal electric arcing, given present techniques and manufacturing equipment. Additionally, such wall thicknesses provide an acceptable level of strength.
However, with conventional single layer EVA films, irradiation doses on the order of 15 to 40 megarads are required to produce the degree of cross-linking necessary to withstand sterilization temperatures. Such high irradiation dosage levels, coupled with the relatively large amounts of EVA materials present, cause undesirably high amounts of acetic acids or other extractable materials. The acetic acid produced is undesirable because it can be absorbed by the contents of the container.
Therefore, when using EVA materials in the medical field, it is desirable to use lower irradiation doses and lesser amounts of EVA materials, thereby producing lesser amounts of acetic acid and other extractable byproducts.
Further, known conventional EVA materials suffer from a further disadvantage in that the extrusion process must be carefully designed and controlled. A lack of control during the extrusion process can result in internal stresses, or "frozen-in" stresses, in the material. At room temperature these stresses may be of minimum consequence. However, when containers formed of internally stressed materials are heated within a sterilizing unit, such as a steam autoclave, the internal stresses can lead to excessive shrinkage or distortion of the heated container. In extreme cases, these internal stresses can even cause the containers to rupture.
In addition, known conventional containers formed of EVA materials have to be positioned carefully during sterilization. Adjacent containers in the sterilizing unit tend to adhere or stick to one another due the melting and recrystallizing of the EVA material, even though it may have been sufficiently cross-linked to prevent distortion.
In addition to all of the above-stated disadvantages, containers made of flexible plastics which are radiation crosslinkable, as are EVA materials, are also highly permeable to water, oxygen, and carbon dioxide. Therefore, such containers simply cannot be used for storing aqueous solutions without employing a barrier overwrap. This only adds to the overall expense of the container, while at the same time detracting from user convenience.
One objective of this invention is to provide a medical grade, transparent, flexible plastic container which is readily fillable and drainable, which does not incorporate materials having plasticizers, and which minimizes migration of materials, such as acetic acid, into the contents of the container.
Another objective of this invention is to provide such a container which is as strong as prior art containers, being able to withstand a six foot drop test when filled.
Still another objective of this invention is to provide such a container which will be less subject to distorting or rupturing, due to internal stresses, when heat sterilized and will exhibit little or no container-to-container tackiness during or after heat sterilization.
Yet another objective of this invention is to provide a container which integrally includes barrier materials, eliminating the need for a separate barrier overwrap, thereby adding to the overall economies and the convenience of use.
Still another objective of this invention is to provide a container which, in addition to the above-stated advantages, can be formed using dielectric radio frequency heat sealing methods, thereby providing stronger fluid resistant seals than are possible with impulse or hot-bar heat sealing methods.